Crossing a Shallow Shoal on a Falling Tide
You are planning a 9 nautical mile passage from a marina to an anchorage behind a shallow shoal. Your boat draws 0.9 m and you want at least 0.5 m under the keel. The chart shows 0.3 m over the shoal at chart datum. The port tide table gives: 0600 0.7 m; 0700 1.2 m; 0800 1.7 m; 0900 2.0 m; 1000 2.3 m; 1100 2.4 m high water; 1200 2.3 m; 1300 2.1 m; 1400 1.8 m; 1500 1.4 m; 1600 0.9 m; 1700 0.4 m. The forecast is 15 knots against the ebb, with a short 0.8 m sea over the shoal. Your comfortable cruising speed is 6 knots. You plan to leave the marina at 0800 and return in the early afternoon. The shoal is 3 nautical miles from the marina and 6 nautical miles from the anchorage. There is no alternative deep-water route. What is your tidal window, and how does the sea state affect your margin?
1. Calculate the tide height needed over the shoal. What are the earliest and latest crossing times with a 30-minute safety margin?
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Required total depth = draught 0.9 m + under-keel clearance 0.5 m = 1.4 m. Tide needed over the 0.3 m charted shoal = 1.4 − 0.3 = 1.1 m. From the table, 1.1 m is reached between 0600 (0.7) and 0700 (1.2): 0.4/0.5 = 0.8 h after 0600, so about 0648. On the falling tide it is reached between 1500 (1.4) and 1600 (0.9): 0.3/0.5 = 0.6 h after 1500, so about 1536. Applying a 30-minute safety margin gives an earliest crossing around 0718 and a latest around 1506. Linear interpolation is approximate; I would round toward more water, not less.
2. Does the planned 0800 departure work? What about a 1400 departure from the anchorage for the return?
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Leaving the marina at 0800 puts you over the shoal at about 0830 (3 nm at 6 kt = 0.5 h). Tide at 0830 is about 1.85 m, so total depth = 0.3 + 1.85 = 2.15 m; clearance = 2.15 − 0.9 = 1.25 m, well above the 0.5 m minimum. The outbound leg is fine. For the return, leaving the anchorage at 1400 puts you at the shoal at 1500 (6 nm at 6 kt = 1 h). Tide is 1.4 m, total depth 1.7 m, clearance 0.8 m. That works on paper, but the 15 kt wind against the ebb and 0.8 m sea will reduce effective clearance through squat and wave troughs. I would keep the return crossing no later than about 1500 and avoid the last of the ebb.
3. You arrive at the shoal at 1545 and the sea is breaking. What do you do?
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At 1545 the tide is about 1.175 m (1.4 − 0.45 × 0.5). Total depth is 1.475 m, so nominal clearance is 0.575 m - only 7.5 cm above the 0.5 m minimum. That is not a real margin with a 0.8 m sea, squat and an ebbing tide still falling. I would not cross. Turn back, anchor or hold off in safe depth, and wait for the next flood; or use a deeper alternative if one exists. The correct decision is based on the worst trough, not the average charted number.